1. Field of the Invention
This invention relates to the protection of batteries from damage due to excessive temperatures generated during charging or discharging and to batteries which are "intrinsically safe".
2. Summary of the Prior Art
When a battery is discharged, and in the case of a secondary battery, also when it is recharged and the battery is approaching a fully charged state, heat is generated within the battery. The higher the current drawn from the battery, or the higher the charging current, the greater the amount of heat generated and the greater the danger that one or more of the components of the battery will be damaged. Batteries do not normally contain any component to prevent excessive currents being drawn from them, though it is known to include a thermal fuse or a thermostat in a battery for this purpose. On the other hand, when a secondary battery is to be recharged by a period of rapid-charging followed by a period of trickle-charging, it is common practice for the battery to incorporate a PTC resistor (i.e. a resistor whose resistance rises with temperature) which is in thermal contact with the cells, e.g. nested between them, and which, when the battery is being charged, forms part of a separate control circuit which contains electronic switching apparatus such that when the resistance of the resistor increases beyond a specific value (as a result of increased battery temperature), the current in the charging circuit is reduced by this control circuit from the rapid-charging level to the trickle-charging level. Charging procedures of this kind are described for example in U.S. Pat. No. 3,667,026. While such procedures in general give satisfactory control over the battery-charging process, they involve the use of excessively complex and expensive electronic switching apparatus.